The present invention relates to systems and methods for generating controllable beam of electrons using a hollow cathode triode electron gun that substantially mitigates the impact of back-streaming of the electrons.
A vacuum electron device (VED), such as a linear particle accelerator or a Klystron, uses a source of an electron beam which is typically known as an electron gun.
Conventional electron guns are of two types. The first type of electron guns is the diode electron gun which has two electrodes; namely a cathode and an anode. The second type of electron guns is the triode electron gun which has three electrodes; namely a cathode, an anode, and a grid.
The triode electron gun has operational advantages over the diode electron gun. One advantage is allowing for fast changes in the electron beam current produced by the electron gun. In the case of the diode electron gun, changing the electron beam current is done by changing a high-voltage difference between the cathode and the anode which is normally thousands of volts. In the case of the triode electron gun, changing the electron beam current is done by changing a voltage difference between the cathode and the grid which is normally a few or less than 100 volts. Thus, changing the electron beam current can be done faster and in a more controlled way.
A major use of a triode electron gun is to supply electron beam current to a linear particle accelerator (Linac). A common problem associated with Linacs is that some electrons entering the Linac's RF Structure are out of synchronism with the RF (electromagnetic energy) and are reflected back towards the electron gun at accelerated velocities and this is commonly called back-streaming electrons. These back-streaming electrons impact its cathode and raise its temperature. The cathode is normally impregnated with a material, such as Barium, that enhances electron emission by lowering the cathode's work function. The rise of the cathode temperature increases the evaporation rate of the impregnating material. Over time this same impregnate material adheres to all surfaces that are line-of-sight, mainly the gun's grid which is directly in front of the cathode's emitting surface. The grid is kept at a voltage very near the same potential voltage as the cathode and thus sees a voltage gradient between it and the anode which is at ground potential. The back-streaming electrons impact the grid, raising its temperature. With the deposit of the impregnating material on the grid and the rise of its temperature due back streaming of electrons, the grid can emit unwanted electrons and in an uncontrolled way.
The back-streaming electrons also impact the center portion of the cathode's emitting surface, raising its temperature and consequently increasing the evaporation rate of the impregnating material. This excess impregnating material will adhere to the grid and can lead to unwanted emission due to high DC field gradients and will also adhere to other line-of-sight surfaces, including the Linac's RF structure that is down-stream from the cathode. The Linac structure also has high RF field gradients and when its surfaces become coated with the impregnating material it would experience field emission of unwanted and uncontrolled electrons which form what is commonly known as “dark current.”
It is therefore clear that an urgent need exists for an improved electron gun that is a triode and can substantially mitigate impact of back-streaming of the electrons and addresses the above described problem of the emission of unwanted and uncontrolled electrons. The present invention is concerned with a triode electron gun. Particularly, relates to a triode electron gun with hollow cathode used with vacuum electron devices (VED's).